1. Field of the Invention
This invention relates to improvements in electronic circuitry of the type used with polyphase dc motors, and, more particularly, to improvements in commutation delay circuits for use with such motors, and, still more particularly, to an improved circuit and method for automatically establishing the commutation delay for producing optimum motor efficiency.
2. Technical Background
In the operation of polyphase dc motors, typically a delay is provided between the occurrence of a motor position indicating event and the commutation of the motor to a next coil energization sequence. The position indicating event can be, for example, a pulse produced on a Hall-effect sensor, a back emf zero crossing in a floating coil, or other event.
Although the theoretical commutation time from the position indicating event generally can be accurately calculated, other factors such as delays in the circuit, motor rotation inhibiting friction, and the like, can result in the actual optimum commutation time being different from that theoretically calculated. As the delay is changed from the optimum value, the amount of current that is provided to the motor to sustain its rotation at a particular speed increases. Also, at the optimum commutation point, the maximum torque of the motor is developed for a minimum amount of current (or energy) delivered to it.
Other factors that affect the delay are non-linearities in the drive circuit at various drive currents. For example, at a first current that maintains a first motor speed, the delay may be different from a second drive current that maintains a second rotational speed. In addition, variations among motors themselves due to the particular coil configurations and other manufacturing differences, produce different optimum commutation delay periods from motor to motor.
In a typical three-phase dc motor, it has been found, for example, that an optimum commutation delay is 30 electrical degrees from a zero crossing of the back emf of a floating coil of the motor. Still, it has been found that typical motor driver delays for such motors are roughly 20 to 22 electrical degrees to result in the 30 degree commutation delay seen at the coils of the motor.
It should be noted that changes in the energy consumption that result from small differences in commutation delay, of say, for example, between 22 and 28 degrees, does not result in a particularly large energy savings. But, considering the increasingly strict demands for energy efficiency, particularly, in laptop computers that rely on rechargeable batteries, or similar environments, even small improvements are increasingly desirable.
In the past, the commutation delay has been typically established by an R-C circuit that is charged from a first zero crossing of the back emf of a floating coil. Then, after a second zero crossing, the capacitor is discharged at twice the rate. When the capacitor becomes discharged, the midpoint of the zero crossings will have been reached. Thus, the commutation may be initiated at that point. Adjustment of the precise commutation time can, therefore, be manually adjusted by varying the discharge time of the capacitor.